Instruments for bed level detection
10. INSTRUMENTS FOR BED LEVEL DETECTION
The management of rivers, estuaries and coastal seas always involves the production of bathymetric maps for evaluation of navigationable depths, shoaling and erosion volumes, etc. Hence, accurate measuring instruments for bed level detection are required. Herein, the following methods and accuracy involved are discussed: mechanical bed level detection in combination with DGPS; acoustic bed level detectors (single and multi-beam echo sounders); and optical bed level detection.
10.2 Mechanical bed level detection in combination with DGPS
In coastal environments the bed level soundings are often performed by use of a vehicle moving through the surf zone. Rijkwaterstaat (The Netherlands) uses the WESP in combination with DGPS. The CRAB vehicle is in use at the Duck site (USA). The WESP is an approximately 15 m high amphibious 3-wheel vehicle, which can be used for bed level soundings in the surf zone in depths up to -6 m with waves upto 2 m. It is equipped with a DGPS positioning system. Small vehicles with DGPS can be used on the dry beach.
10.3 Acoustic bed level detection (Echo-sounding instruments)
The most common system for measuring water depth is the single-beam echo sounder. This sonar instrument uses a transducer that is usually mounted on the bottom of a ship. Sound pulses (usually 210 KHz for surface detection) are sent from the transducer straight down into the water. The sound reflects off the seafloor and returns to the transducer. Acoustic penetration into the bed increases with decreasing frequency (usually 10 to 15 KHz for subsurface detection). The time the sound takes to travel to the bottom and back is used to calculate the distance to the seafloor. Water depth is estimated by using the speed of sound through the water (approximately 1500 meters per second) and a simple calculation: distance = speed x time. The faster the sound pulses return to the transducer from the ocean floor, the shallower the water depth is and the higher the elevation of the sea floor. The sound pulses are sent out regularly as the ship moves along the surface, which produces a line showing the depth of the ocean beneath the ship. This continuous depth data is used to create bathymetry maps of the survey area.
Multibeam bathymetry sonar (Figure 2) is the relatively recent successor to single-beam echo sounding. About 30 years ago, a new technology has been developed that uses many beams of sound at the same time to cover a large fan-shaped area of the ocean floor rather than just the small patch of seafloor that echo sounders cover. These multibeam systems can have more than 100 transducers, arranged in precise geometrical patterns, sending out a swath of sound that covers a distance on either side of the ship that is equal to about two times the water depth. All of the signals that are sent out reach the seafloor and return at slightly different times. These signals are received and converted to water depths by computers, and then automatically plotted as bathymetric maps.
One of the best systems for imaging large areas of the ocean floor is side scan sonar (Figure 3A), either ship-mounted or bottom-mounted. The basic concept is much the same as the basic echo sounder; however, side scan sonar instruments are towed behind ships and often called towfish or tow vehicles. This technology uses a specially shaped acoustic beam, which pulses out 90 degrees from the path that it is towed, and also out to each side. Each pulse provides a detailed image of a narrow strip directly below and to either side of the instrument.
Seismic reflection uses a stronger sound signal and lower sound frequencies than echosounding. The sound pulse is often sent from an airgun towed behind the ship. An airgun uses the sudden release of compressed air to form bubbles. The bubble formation produces a loud sound. The sound from the airgun travels down to the seafloor. Some of the sound reflects off the seafloor but some of the sound penetrates the seafloor. The sound that penetrates the seafloor may also reflect off layers of material within the seafloor. The reflected sounds travel back up to the surface. The ship also tows a number of hydrophones (called a towed array or streamer) which detects the reflected sound signal when it reaches the surface. The time it takes the sound to return to the ship can be used to find the thickness of the layers in the seafloor and their position (sloped, level, etc). It also gives some information about the composition of the layers.
10.4 Optical bed level detection
This instrument consists of a steel pole (diameter of 32 or 40 mm; lengths of 1.8, 2.4 and 2.9 m), which can be driven into the bed. The pole is supplied with many infra-red light sources/receivers (backscattering sensors) at spacings of 10 mm (100 sensors per meter; sampling volume of 0.5 cm3).
The instrument measures:
- vertical distribution of the turbidity levels in the water column;
- transition from water column to bed based on the scattering of light from the suspended particles and the bed material particles;
- transition from water column to air (if pole end is above the water surface).